Lens

A lens (here: photographic lens, also known as objective lens or photographic objective) is an optical device through which light is focused in order to form an image inside of a camera either on film or on a digital sensor.

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An element of a photographic lens is an optical lens itself, but one consisting of one single round piece of honed glass or plexiglass. Most photographic lenses have several different elements combined in the lens barrel, all having the same optical axis. Two or more elements which are cemented together (without an air space between them) make a group. (Older lenses used Canada balsam, made from pine pitch, for cement, while newer lenses use higher-tech UV-cured cements.)

Making a lens element is not as simple as just having a piece of founded glass. That's just the beginning. After having founded a an appropriate piece of glass its surfaces must be ground until their plane or spherical concave or spherical convex surfaces are according to the lens elements' mathematically calculated geometry. If a surface has to be aspherical the grinding process becomes more complicated. After grinding the surfaces must be polished since they must be transparent. Nowadays one, some or even all element surfaces of a lens get a transparent special coating against reflections between the elements. The elements are classified as converging (light-bundling) and diverging (light-spreading) elements

The basic materials of lens elements is either optical glass or transparent plastic material like acrylic glass or plexiglass. Some microscopes use oildrop lenses, and some lens constructions have elements filled with water. An exception are the mirror elements of some super-tele lenses which can be made of other materials.

Flint glass is made with a high allotment of lead. Its prismatic division of colors is quite different from that of crown glass. That effect made the construction of color-corrected lenses possible by combining elements of both types of glass so that each neutralizes the prismatic color dispersal effects of the other. The first renowned achromatic lens construction was found 250 years ago by the English optician John Dollond on the basis of his own research and maybe little help of the Swedish expert Samuel Klingenstierna.

The aperture is a round opening in or behind a lens (or between elements of it) that limits the amount of light passing through it into the camera. The lens's optical axis passes in 90 degree angle through the centre of that hole. The term aperture is also often used to describe the amount of light transmitted by the lens, for which the size of this opening is one determining factor. It is usually adjustable, automatically or by turning an aperture control, usually but not always on the lens barrel. A diaphragm is a round aperture of variable size.

The focal plane is the plane onto which a lens projects the image of the focused image subject. Usually it's flat but especially some old bakelite cameras with just a simple meniscus lens have a curved image plane since the "curvature of field" of such lenses is stronger than that of more sophisticated multi-element lenses which deliver more or less "planar" images. Usually the middle of a focal plane sits at a 90-degree angle to the optical axis, except when tilt/shift movements cause deviations from a camera's normal geometry of light-pathes. The position of the film's or digital sensor's light sensitive surface should be identical with the focal plane.

A fixed lens is simply a lens that is permanently fastened to its camera as opposed to a system camera that allows different lenses to be used on the same camera easily. Fixed lenses are commonly found cameras aimed at consumers, from old box. There are certain advantages to having a fixed lens on your camera:
As no mechanism for changing lenses needs to be built into the camera design it can help keep the camera smaller and lighter. A fixed lens means that there is less chance of introducing dust to the sensor surface.Fixed lenses are designed for a specific camera model and so fewer compromises have to be made in the lens design. Cost - if your camera comes with a fixed lens you don't have to worry amount buying a lot of additional glass to build a system. Portability - a fixed lens should be enough for most situations you encounter so you have less accessories to carry and you will waste less time changing lenses.

Interchangeable lenses are more commonly found on cameras aimed at professionals and enthusiasts including large format. The advantages to interchangeable lenses include:
A larger range of focal lengths and specialties (shift, macro, etc.) are available than you are likely to find on any fixed lens camera. Each lens can be designed for a specific kind(s) of working situations and specialties without the compromises a generalist fixed lens has to be designed for. Longevity - you can upgrade your camera body without losing any investment you have made in additional lenses if your new camera choice is in the same family as your old camera.</

A given interchangeable lens body can accept one type of lenses. There are cases of compatibility, when different bodies share the same. It can exist to put a lens designed with one type of lens mount on a body designed for another. Auxiliary lenses - if your camera has a fixed lens there are accessories available that allow you to enhance your fixed lenses range. These included close-up lenses that allow your camera to focus closer than it naturally can. They also include wide-angle attachments that allow your fixed lens to capture more of a scene than it otherwise could. They also include popular telephoto attachments that allow your fixed lens to reach further than it otherwise could. Telephoto attachments include extreme digiscoping lenses. Digiscoping is the practice of mounting a digicam on a spotting scope of telescope to create extreme focal lengths.As with all photographic equipment, auxiliary lenses range in quality from the truly dreadful to the professional. Bear in mind that any auxiliary lens that you attach to your fixed lens is adding more glass between the subject and the film. As such it is bound to affect image quality and the amount of light passing through to the film plane. Cheap auxiliary lenses add horrible distortion and purple fringing to your shots. Auxiliary lenses are a compromise solution to extending the range of a fixed lens that can provide good results but there appear to be no bargains in this niche marketplace and you will get what you pay for.

The term prime refers to a lens with a single focal length. Typically, prime lenses (except repro lenses) have larger maximum apertures, so they are able to let in more light wide open than similar zoom lenses. This makes prime lenses more suitable to low-light photography. Except for ultra-wide-angle lenses, prime lenses give images with less distortion.

A zoom lens is a compound lens with a variable effective focal length. While (contrary to a popular misconception) the perspective does not change, shifting the focal length of a zoom lens does allow the photographer to modify the crop of a photo without moving. Zoom lenses are bulkier than fixed lenses, but they introduce an extra adjustment you can make before taking the picture. The vast majority of digital cameras come equipped with Zoom lenses.

The zoom ratio is the ratio between the shortest focal length and the longest focal length of a given lens. The majority of modern zoom lenses are about 1:3, meaning that their longest focal length is 3 times the shortest. For example, there are many 35-105 lenses available. As the ratio gets bigger, the lens becomes much harder to manufacture, and more expensive. Some modern digital cameras have zoom ratios of 1:10, or even 1:12. It may be that such a camera could lessen the need for interchangeable lenses, and perhaps these will become more of the norm. Currently, they represent the leading edge of consumer optical technology.

Frequently, lenses for digital cameras are labelled with the focal length they would have if they were 35mm cameras. This gives a way of comparing zoom ratios between film and digital cameras. In any case, divide the larger number by the smaller. If the result is less than 3, then it's unimpressive. If it's about 3, then it's a normal, conservative design. If it's much greater than 4, some testing might be in order.

In general, the shorter the focal length of the lens, the wider it's angle of view. It's much easier to get very wide angle lenses for film cameras than for digital, unless you are talking about very expensive digital cameras. Many digital cameras suffer from a lack of wide angle ability, and if that's important to you, that will affect your choice. If you must have very wide angles, you will need to get a camera with interchangeable lenses, whether film or digital. Some lower cost digital cameras can be fitted with add-on lenses that increase their angle of view. In 35mm cameras, a 50mm lens is said to give an angle of view similar to the human eye, though many people dispute this. Nevertheless, this has come to be called a standard lens for 35mm cameras.

A normal lens has an angle of view that approximates how the human eye sees a scene. A lens is considered normal when its focal length is approximately equal to the diagonal of the film format. Lenses shorter than normal are called wide-angle, while those longer are called telephoto. In 35mm photography, 50mm is considered to be the normal focal length, even though the actual diagonal of the frame (24mm x 36mm) is 43mm. For medium format photography (frame size 2-1/4" square, or 6x6cm), normal is generally 80mm.

A wide-angle lens is a lens with a focal length shorter than normal. A wide-angle lens, as opposed to a fisheye, is normally well-corrected for geometrical distortion, i.e., straight lines appear straight. However, as the angle of view is wider than that seen by the human eye, they can create an impression of distortion; this perceived distortion increases as the focal length of the lens decreases. A fisheye lens (see below) is an extreme example of a wide-angle lens.

A long-focus lens is any lens with a focal length longer than normal. A long-focus lens brings far subjects closer, like a telescope. They therefore have a smaller angle of view than a normal or wide-angle lens. The longer the lens, the more likely that camera shake will blur the image; for this reason, longer lenses are usually used with a tripod to steady the camera.

A telephoto lens is a particular optical construction used in the design of many long-focus lenses. A simple telephoto lens has a convex lens group at the front and a concave lens group at the rear. This optical design results in lens with a physical length shorter than the net focal length.

Fisheye lenses have the widest field of view of any lens group. The geometrical projection is far different from the classic perspective we are used to, as discussed here, and straight lines appear curved if they are near the edge of the image. This creates distortion of the resulting image in a dramatic way. Fisheye lenses fall into two categories:

Circular fisheyes: have a 180 degree field of view when measured along the smallest dimension of the image, resulting in a circular image with black corners. In 35mm format, they usually have a focal length around 8mm.

Full frame fisheyes: have a 180 degree field of view when measured along the diagonal, so the image extends on the full film plane. In 35mm format, they usually have a focal length around 16mm.

In general, fisheye lenses are expensive and little used in everyday photography.
They are used for measuring, scientific research tasks, especially meteorology, illumination studies, botanic studies on trees.
Another important application field is panoramic and VR photography

Fisheye adaptors: auxiliary lenses are available that simulate a fisheye field of view. This is a cheap way to play with the fisheye effect without investing in a dedicated fisheye lens, although, as with most auxiliary lenses, the quality of your images will not be the same as those taken using a 'real' fisheye.

Macro lenses are lens heads for SLR's supplementary bellows, or belong to the macro subclasses of wide-angle lenses, normal lenses, mainly zoom lenses, and even tele lenses (telemacros).
Macro means that macroscopic exposures are possible since these lenses allow very near image subject distances. Wide-angle lenses may allow distances of 20 cm. On most zoom lenses near distances cannot be chosen directly. Those lenses have to be switched to a special macro mode. Modern digicam zoom lenses have macro modes for minimal image subject distances between 1 (!) and 10 cm. Lenses with a longer tube elongation added may allow shorter distances. Such elongations are usually reached with supplementary macro bellows or macro rings. On top of a macro bellows nearly any sort macro lens head, normal lens, wide-angle lens or zoom lens can be used for macro photography, even if the lens is not explicitly sold as macro lens. For macro lenses or lens/elongation combinations the maximum reproduction scale (or reproduction ratio) is a characteristic parameter. For example a reproduction scale of 1:3 means that the object focused in shortest distance will be reproduced in one third of its original size. Only on a print will it appear enlarged. A reproduction scale of 2:1 means that the object focused at the shortest focusing distance will be reproduced in twice its original size. It will appear enlarged on the negative.

Repro lenses are not constructed for speed but for sharpness. Many are made for making images of frame sizes greater or equal 9×12cm. And many allow short subject distances. These lenses are ideal for reproducing two-dimensional objects. Used on long bellows repro lenses are good macro lenses. Another usage is making images of still objects in studios. Naturally repro cameras need this sort of lens. Many repro lenses, even some for smaller frame formats, are constructed apochromatic.

Some lenses are optical adapters allowing to use cameras with microscopes, endoscopes or telescopes. Others are converters to be mounted between camera and lens to give the lens twice of its original focal length.

Lenses that let in a lot of light are called 'fast' lenses. This quality is indicated by a number which is called the maximum aperture or maximum f stop. The smaller the number, the faster the lens.

"Fast lens" is a relative description: It depends on the image format and the focal length. Considering 35mm film 24x36mm frame format, a fast lens with fixed focal length between 24mm and 135mm will typically have a maximum aperture of f2.5 or less. f2.8 200mm lenses and f3 300mm lenses are fast lenses, while the same focal lengthes combined with maximum aperture of f2.0 are exceptionally super fast. Fast zoom lenses have a maximum aperture of at least f2.8 plus max. aperture f4.5 in tele mode.

f1.8 for focal length 50mm and f2.8 for other focal lengthes between 24mm to 135mm are the standard lens speeds for 35mm format lenses. f2.8 is already fast lens speed when regarding medium format lenses. f1.4 is a standard lens speed for 16mm movie camera lenses.

The max. aperture number or lens speed is equal to the largest aperture's diameter divided by the focal length of the lens. As lenses get faster, they become larger, more difficult to make, and more costly. Fast lenses are important if you want to take photos in dim light without flash, and without a tripod or if you need a shallow depth of field.

The depth of field is a way of describing how much of your image is in focus. When the camera is focused at a certain point, it will remain in focus for objects slightly in front of that point, as well as slightly behind. The distance between the closest object that is in focus, and the most distant one, is the depth of field.

The depth of field is dependent purely on the geometry of the lens, and cannot be changed by the manufacturer.

Generally, the shorter the focal length of the lens, the greater its depth of field. Zoom lenses have more depth of field when set to their shortest focal length, than when set to the longest. Since all small digital cameras have lenses with very short focal lengths, they tend to have very large depth of field. This has many benefits, and generally makes the job of the autofocus mechanism much easier. On the other hand, certain aesthetic effects become more difficult when the lens has too much depth of field, for example having a sharp subject emphasized over a blurry background.

Finally, the faster the lens, the lower the depth of field. This means that while using a very fast lens will allow you to photograph in dim light, it will be very difficult to adjust the focus when you do this. Moral: if you think you want a very fast lens, you will pay for it in cost, weight, bulk, and poor depth of field. But of course you will be able to take pictures in dim light, and to detach your subject from the background in more situations.

This is when the lens represents straight lines as bent. This can be often seen in zoom lenses at both ends of the zoom range, straight lines at the edge of the frame will appear slightly curved. There seems to have arisen a kind of accepted dogma that this is a bad thing, though nobody seems to explain why. In fact it depends on the type of pictures you will take. It will usually be bad with architectural pictures, with pictures full of geometrical shapes, and for the reproduction of flat objects such as paintings. It will usually not matter for portraits and for daily shooting. If you think it matters for you, then check carefully for that behavior in any lens you are choosing. Once again, it seems to be difficult for lens manufacturers to achieve very low distortion in conjunction with all the other good features they want their lenses to have. The lenses on which distortion seems the most difficult to correct are the wide angle lenses.

Astigmatism (pointlessness) is when a point sending light through a lens cannot be projected as one point behind the lens. It appears as a line on the focal plane. Another explanation is that astigmatic lenses cannot project horizontal lines into the same image plane as vertical lines. That effect mainly appears when biconvex or biconcave lens elements are used. In the case of the biconvex lenses in our eyes astigmatism can be corrected by a lens element with reverse astigmatic effect (cylinder lens). The only possible correction of astigmatism of camera lenses is to combine at least 3 lens elements. The different elements of a well-constructed triplet minimize astigmatism. For more than one hundred years most photographic lenses are anastigmatic, but the term was still used for marketing camera lenses until the 1950s.

Chromatic aberrations are reduced by using elements made of different varieties of glass. Elements made of different glass help to bundle red, green and blue light that is coming from one single point in front of the lens in one single point behind the lens. Lens designs which are projecting blue and green light focused unified are called achromatic lenses. When the focusing correction of the lens includes blue, green and red light it's an apochromatic lens design.

The combination of two convex lens elements made of crown glass with a concave

one which is made of flint glass is known since 1765 as a means against chromatic
aberration. Achromatic lenses enable well focused projection of blue and green
light. The draft exaggerates the different optical paths of the blue and
the green light shares of the light coming from a blue/green (cyan) light mixture.

Modern lenses are coated with a very thin layer of antireflective material (like magnesium fluoride or calcium fluoride). This lens coating is applied to the each element of a lens which has a surface exposed to air. The only purpose of coating lenses is to reduce lens flare by eliminating reflection off the surface of the glass; this has the effect of increasing contrast and giving images more "punch". Lens coating thicknesses are typically of the order of a few wavelengths of light, - a few tens of nanometers (nm). Lens coatings have nothing to do with color correction of lenses, as is widely thought. An uncoated surface will exhibit a reflection ~4% of the incident light, in a three element lens (with six surfaces) this represents a transmittance of ~78%. A Magnesium fluoride coated surface would exhibit a reflectivity of ~1% so the six surface system may have a transmittance of ~94%.

Multicoating refers to the application of more than one layer of coating on a lens. Multicoated lenses may have might higher transmittances that single coated lenses, and may be specifically tailored for use with a small number of frequencies. Coatings on lenses may used to enhance scratch resistance.

All modern lenses are coated. Coating gives lenses their colored reflective look. Since lens coatings are relatively fragile, one should always be careful when cleaning coated lens surfaces (like the front of the lens), so as not to scratch or smudge them. Many photographers keep a daylight or UV filter on the lens to protect its surface and avoid the necessity of frequent cleaning.

According to this source [1] of Rochester University ("Wither Optical Design?", by Douglas Sinclair) lens design reached its heyday in the 20th century. It was the task of the "traditional" lens designers to "balance the aberrations of centered optical systems to achieve a maximum image quality". The optical engineer "worked on layouts and negociated with the marketing and mechanical departments to get enough room for the design to be implemented within the laws of physics." On the one hand Sinclair predicted that the optical engineering will take over the lens designers' tasks in the 21st century. On the other hand he reported from the 1998 lens design contest of the renowned International Optical Design Conference that the top-five designs were made by experienced lens designers. All of them used lens design software. Sinclair's prediction may be wrong: The contest proved that modern lens designers can work with software tools which may be developed by optical engineers, but as Sinclair himself wrote the good lens designs are more than the software can generate. According to his writings it's almost an art to add practical lens design experience to purely technical solutions. What may have changed is that both, optical engineers and lens designers need broader understanding of the whole related fields of physics, geometrical optics, wave optics, algorithms, etc. .